Light weight high temperature well cement compositions and methods

ABSTRACT

The present invention provides methods of cementing subterranean zones using cement compositions comprising calcium aluminate, fly ash, sodium polyphosphate and water.

CROSS-REFERENCES TO RELATED APPLICATIONS

[0001] This application is a Continuation of Ser. No. 09/635,603 filedAug. 9, 2000, pending, which is a Continuation of Ser. No. 09/123,004filed Jul. 27, 1998, U.S. Pat. No. 6,143,069, which is aContinuation-In-Part of Ser. No. 08/912,203 filed Aug. 15, 1997, U.S.Pat. No. 5,900,053.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates generally to light weight hightemperature well cement compositions and methods, and more particularly,to such compositions and methods which are suitable for cementing hightemperature wells containing carbon dioxide.

[0004] 2. Description of the Prior Art

[0005] In the completion of high temperature subterranean wellscontaining carbon dioxide, e.g., geothermal wells, the use ofconventional hydraulic cement compositions often results in early wellfailure. Because of the high static well bore temperatures involvedcoupled with the presence of brines containing carbon dioxide,conventional hydraulic well cements rapidly deteriorate due to alkalicarbonation, especially sodium carbonate induced carbonation. Ingeothermal wells, which typically involve very high temperatures,pressures and carbon dioxide concentrations, conventional well cementfailures have occurred in less than five years causing the collapse ofthe well casing.

[0006] It has heretofore been discovered that a cement material known ascalcium phosphate cement formed by an acid-base reaction between calciumaluminate and a phosphate-containing solution has high strength, lowpermeability and excellent carbon dioxide resistance when cured inhydrothermal environments. However, calcium phosphate cement has arelatively high density, e.g., a density in the range of from about 15to about 17 pounds per gallon, which is too high for geothermalapplications. That is, in geothermal wells the hydrostatic pressureexerted by the high density calcium phosphate cement often exceeds thefracture gradients of subterranean zones penetrated by the well borewhich causes the formation of fractures into which the cement is lost.While calcium phosphate cements have been developed which include hollowmicrospheres and as a result have densities of about 10 pounds pergallon, such light weight compositions are relatively expensive and thepresence of the microspheres in the cured cement reduces its compressivestrength.

[0007] Thus, there is a need for improved less expensive well cementcompositions useful in cementing high temperature wells containingcarbon dioxide.

SUMMARY OF THE INVENTION

[0008] The present invention provides improved cement compositions andmethods which meet the needs described above and overcome thedeficiencies of the prior art. The compositions are particularly usefulin high temperature wells containing carbon dioxide such as geothermalwells. A composition of the present invention is basically comprised ofcalcium aluminate, fly ash and sufficient water to form a pumpableslurry.

[0009] Another composition of this invention is comprised of calciumaluminate, fly ash, sufficient water to form a pumpable slurry, afoaming agent, a foam stabilizer and a gas sufficient to form a foamhaving a density in the range of from about 9.5 to about 14 pounds pergallon.

[0010] Yet another composition of this invention is comprised of calciumaluminate, sodium polyphosphate, fly ash, sufficient water to form apumpable slurry, a foaming agent, a foam stabilizer and a gas present inan amount sufficient to form a foam having a density in the range offrom about 9.5 to about 14 pounds per gallon.

[0011] The methods of the present invention for cementing a hightemperature subterranean zone containing carbon dioxide penetrated by awell bore basically comprise the steps of forming a well cementcomposition of this invention, pumping the cement composition into thesubterranean zone by way of the well bore and allowing the cementcomposition to set into a hard impermeable mass therein.

[0012] It is, therefore, a general object of the present invention toprovide light weight high temperature well cement compositions andmethods.

[0013] A further object of the present invention is the provision ofimproved carbonation resistant well cement compositions and methods.

[0014] Other and further objects, features and advantages of the presentinvention will be readily apparent to those skilled in the art upon areading of the description of preferred embodiments which follows.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0015] As mentioned above, high temperature wells containing carbondioxide such as geothermal wells generally require the use of wellcement compositions which do not deteriorate in the presence of carbondioxide containing brines. The term “high temperature” is used herein tomean wells wherein the static bottom hole temperature is above about300° F. up to as high as about 700° F. When conventional hydrauliccements are utilized in such wells, carbonation causes dissolution ofthe cement which is converted into water-soluble salts. Further, severecorrosion of steel pipe takes place thereby resulting in the totaldisruption of the conventional cement supported well structure.

[0016] When conventional normal density cement slurries are utilized ingeothermal and other similar wells, loss of circulation problems areoften encountered. This is due to the weak unconsolidated formations inthe wells having very low fracture gradients. When a relatively highdensity cement slurry is pumped into such a well, the hydrostaticpressure exerted on the weak unconsolidated subterranean zones thereincauses the zones to fracture. This in turn causes the cement slurrybeing pumped to enter the fractures and lost circulation problems tooccur. To avoid such problems, the cement compositions utilized ingeothermal and other similar wells must be of light weight, i.e., havedensities in the range of from about 9.5 to about 14 pounds per gallon.

[0017] By the present invention, improved well cement compositions areprovided which resist high temperature carbonation deterioration. Acement composition of this invention which can be non-foamed or foamedis basically comprised of calcium aluminate, fly ash and sufficientwater to form a pumpable slurry. When foamed, the cement compositionincludes a foaming agent, a foam stabilizer and a gas present in anamount sufficient to form a foam having a density in the range of fromabout 9.5 to about 14 pounds per gallon.

[0018] Another composition of this invention is comprised of calciumaluminate, sodium polyphosphate, fly ash, a foaming agent, a foamstabilizer and a gas present in an amount sufficient to form a foamhaving a density in the range of from about 9.5 to about 14 pounds pergallon.

[0019] The calcium aluminate can be any commercial grade calciumaluminate suitable for use as a cement. A suitable such calciumaluminate is commercially available from the Lehigh Portland CementCompany of Allentown, Pa., under the trade designation “REFCON™.” Thecalcium aluminate is generally included in the cement composition in anamount in the range of from about 15% to about 45% by weight of thecomposition.

[0020] When used, the sodium polyphosphate includes sodium metaphosphateand sodium triphosphate as well as vitreous sodium phosphates. Asuitable sodium polyphosphate for use in accordance with the presentinvention is commercially available from Calgon Corporation ofPittsburgh, Pa. The sodium polyphosphate can be included in the cementcomposition in an amount in the range of from about 5% to about 20% byweight of the composition. When included, the sodium polyphosphatecombines with the calcium aluminate to form calcium phosphate in theform of hydroxyapatite.

[0021] Fly ash is the finally divided residue that results from thecombustion of ground or powdered coal and is carried by the flue gasesgenerated. A particular fly ash that is suitable in accordance with thepresent invention is a fine particle size ASTM class F fly ash having aBlaine fineness of about 10,585 square centimeters per gram which iscommercially available from LaFarge Corporation of Michigan under thetrade designation “POZMIX™.” Another fly ash that is suitable is an ASTMclass F fly ash which is commercially available from Halliburton EnergyServices of Dallas, Tex. under the trade designation “POZMIX™ A.” Thefly ash is generally included in the composition in an amount in therange from about 25% to about 45% by weight of the composition.

[0022] The major crystalline phase of ASTM class F fly ash is mullite(3Al₂O₃.2SiO₂). It reacts with calcium aluminate to form calcium aluminosilicate (CaO.Al₂O₃.2SiO₂). Also, iron and quartz in the fly ash reactwith the calcium aluminate to form andradite (Ca₃Fe₂SiO₄)₃. Thesereactions increase the compressive strength of the set cement ascompared to set calcium aluminate cement alone.

[0023] The water utilized can be from any source provided it does notcontain an excess of compounds that adversely affect other compounds inthe cement composition. For example, the water can be fresh water orsaltwater. Generally, the water is present in the cement composition inan amount sufficient to form a pumpable slurry, i.e., an amount in therange of from about 10% to about 60% by weight of the composition.

[0024] In order to facilitate the foaming of the cement composition, afoaming agent is included in the composition. A particularly suitableand preferred such foaming agent is an alpha-olefinic sulfonate havingthe formula

H(CH₂)_(n)—CH═CH—(CH₂)_(m)SO₃Na

[0025] wherein n and m are individually integers in the range of fromabout 6 to about 16. The foaming agent is generally included in thecement composition in an amount in the range of from about 1% to about2% by weight of the water in the composition. The most preferred foamingagent of this type is an alpha-olefinic sulfonate having the aboveformula wherein n and m are each 16, i.e., a sulfonic acid C₁₆₋₁₆ alkanesodium salt.

[0026] A foam stabilizer is also included in the cement composition toenhance the stability of the composition after it is foamed. Aparticularly suitable and preferred stabilizing agent is anamidopropylbetaine having the formula

R—CONHCH₂CH₂N⁺(CH₃)₂CH₂CO₂ ⁻

[0027] wherein R is a radical selected from the group of decyl, cetyl,oleyl, lauryl and cocoyl. The foam stabilizer is generally included inthe cement composition in an amount in the range of from about 0.5% toabout 1% by weight of the water in the composition. The most preferredfoam stabilizer of this type is cocoylamidopropylbetaine.

[0028] The gas utilized to foam the composition can be air or nitrogen,with nitrogen being the most preferred. The amount of gas present in thecement composition is that amount which is sufficient to form a foamhaving a density in the range of from about 9.5 to 14 pounds per gallon,most preferably 12 pounds per gallon.

[0029] In order to provide resiliency to the set cement composition ofthis invention, the composition may optionally include inert groundrubber particles. Such particles are produced from worn out tires andare commercially available from Four D Corporation of Duncan, Okla.

[0030] At static well bore temperatures above about 125° F., a setretarder is required. The set retarder functions to lengthen the time inwhich the cement composition starts to thicken and set so that thecomposition can be pumped into the well bore and into the zone to becemented before such thickening takes place. Preferred such setretarders for use in accordance with this invention are gluconic acidand citric acid. When used, the set retarder is included in the cementcomposition in an amount in the range of from about 0.5% to about 2% byweight of the composition.

[0031] A preferred composition of the present invention is comprised ofcalcium aluminate present in an amount of about 30% by weight of thecomposition, ASTM class F fly ash present in an amount of about 50% byweight of the composition and water present in an amount sufficient toform a slurry.

[0032] Another preferred composition of the present invention iscomprised of calcium aluminate present in an amount of about 30% byweight of the composition, ASTM class F Fly Ash present in an amount ofabout 50% by weight of the composition, sufficient water to form apumpable slurry, a foaming agent comprised of a sulfonic acid C₁₆₋₁₆alkane sodium salt present in an amount of about 1.5% by weight of thewater in the composition, a foam stabilizer comprisingcocoylamidopropyl-betaine present in an amount of about 0.75% by weightof the water in the composition and a gas present in an amountsufficient to form a foam having a density in the range of from about9.5 to about 14 pounds per gallon.

[0033] Yet another preferred composition of this invention is comprisedof calcium aluminate present in an amount of about 28% by weight of thecomposition, sodium polyphosphate present in an amount of about 19% byweight of the composition, ASTM class F fly ash present in an amount ofabout 49% by weight of the composition, sufficient water to form apumpable slurry, a foaming agent comprised of a sulfonic acid C₁₆₋₁₆alkane sodium salt present in an amount of about 2% by weight of thewater in the composition, a foam stabilizer comprisingcocylamideopropylbetaine present in an amount of about 1% by weight ofthe water in the composition and a gas present in an amount sufficientto form a foam having a density in the range of from about 9.5 to about14 pounds per gallon.

[0034] As previously mentioned, the above described cement compositionscan include ground rubber particles present in an amount in the range offrom about 10% to about 40% by weight of the compositions to improve theresiliency of the compositions. Further, when the static well boretemperature is above about 125° F., a set retarder selected from thegroup of gluconic acid and citric acid is included in the cementcompositions in an amount of about 1.0% by weight of the compositions.

[0035] The cement compositions of this invention may be prepared inaccordance with any of the mixing techniques utilized in the art. In onepreferred method, a quantity of water is introduced into a cementblender followed by the sodium polyphosphate (if used), calciumaluminate and fly ash. The mixture is agitated for a sufficient periodof time to form a pumpable non-foamed slurry.

[0036] When the cement slurry formed as above is foamed, the slurry ispumped to the well bore and the foaming agent and foam stabilizerfollowed by the gas utilized are injected into the slurry on the fly. Asthe slurry and gas flow through the well bore to the location where theresulting foamed cement composition is to be placed, the cementcomposition is foamed and stabilized. Other liquid additives utilized,if any, are added to the water prior to when the other components of thecement composition are mixed therewith and other dry solids, if any, areadded to the water and cement prior to mixing.

[0037] The methods of this invention of cementing a high temperaturesubterranean zone containing carbon dioxide penetrated by a well boreare basically comprised of the steps of forming a foamed cementcomposition of this invention, pumping the foamed cement compositioninto the subterranean zone to be cemented by way of the well bore andthen allowing the foamed cement composition to set into a hardimpermeable mass therein.

[0038] To further illustrate the improved cement compositions andmethods of this invention, the following examples are given.

EXAMPLE 1

[0039] In a controlled test, API Class G Portland Cement was mixed with40% silica flour and water to form a cement slurry. The slurry wasallowed to set for 24 hours at a temperature of 190° F. Thereafter, theset cement was placed in an aqueous 4% by weight sodium carbonatesolution for 28 days at 600° F.

[0040] A calcium phosphate cement composition was prepared comprised of23.3% water; 17.5% calcium aluminate; 15.6% sodium polyphosphate; 40.8%ASTM class F fly ash, 1.9% sulfonic acid C₁₆₋₁₆ alkane sodium saltfoaming agent and 0.9% cocoylamidopropylbetaine foam stabilizer, all byweight of the composition. After mixing, the resulting slurry wasallowed to set for 24 hours at a temperature of 190° F. Thereafter, theset cement was placed in a 4% by weight aqueous sodium carbonatesolution for 28 days at 600° F.

[0041] At the end of the test periods, samples from the interiors of theset Portland Cement composition and calcium aluminate cement compositionwere tested. The tests showed that the Portland Cement compositioncontained 1.5% by weight calcium carbonate and the calcium phosphatecement contained none. Samples were also tested taken from the exteriorsof the set cements which showed that the Portland cement compositioncontained 10.6% calcium carbonate while the calcium phosphate cementcontained none.

EXAMPLE 2

[0042] Test calcium phosphate cement slurry samples were prepared bymixing 240 grams of water with 180 grams of calcium aluminate, 160 gramsof sodium polyphosphate and 420 grams of fly ash for each sample.Various Portland cement set retarding additives were combined with thetest samples. After mixing, each test sample was tested for thickeningtime at 125° F. in accordance with the test procedure set forth in APISpecification For Materials And Testing For Well Cements, APISpecification 10, 5th ed., dated Jul. 1, 1990 of the American PetroleumInstitute. The set retarders tested are identified and the thickeningtime test results are set forth in Table 1 below. TABLE I ThickeningTime Tests¹ Amount Added to Thickening Time, Set Retarder Tested TestSample, grams hrs:mins. None — 1:35 Acrylic Acid Polymer 6 2:02 TartaricAcid 6 1:12 Gluconic Acid 6 4:05 Citric Acid 6   6:00+

[0043] From Table I, it can be seen that gluconic acid and citric acidare the most effective set retarders for the calcium aluminate cementcomposition at a temperature of 125° F.

EXAMPLE 3

[0044] Two additional calcium aluminate cement slurry samples wereprepared as shown in Table II below. After mixing, the resultingslurries were allowed to set for 24 hours at 190° F. Thereafter, the setsamples were placed in 4% by weight aqueous sodium carbonate solutionsfor 28 days at 600° F. At the end of the 28 day periods, the sampleswere tested for compressive strengths in accordance with the abovementioned API Specification 10. The results of the tests are also setforth in Table II below. TABLE II Compressive Strength Tests SampleComponents, grams Compressive Sample Calcium Sodium Fly Foaming FoamDensity Strength, No. Water Aluminate¹ Phosphate² Ash³ Agent⁴Stabilizer⁵ lb/gal. psi 1 465.5 350 311.5 815.5 37.3 18.6 12.1 570 2 266200 178 466 — — 15.1 1060

[0045] From Table II, it can be seen that the calcium aluminate cementcompositions of the present invention maintained their compressivestrengths after 28 days in the presence of sodium carbonate solutions at600° F.

EXAMPLE 4

[0046] An API Class G Portland cement was mixed with 40% silica flourand water to form a cement slurry. The slurry was allowed to set for 48hours at a temperature of 500° F. Thereafter, the set cement was placedin an aqueous solution containing 2.4% dry ice and 0.8% sulfuric acid. Acalcium aluminate cement composition was prepared comprised of 30%water, 37% calcium aluminate and 33% ASTM class F fly ash. The resultingslurry was allowed to set for 48 hours at a temperature of 500° F.Thereafter, the set cement was placed in an aqueous solution containing2.4% dry ice and 0.8% sulfuric acid. The above described test cementsamples were kept in the carbonate-acid solutions for 53 days at 500°F., after which the Portland cement lost 33% of its weight while thecalcium aluminate cement gained 9.1% in weight.

[0047] Calcium aluminate (Lehigh “REFCON™”) was mixed with 59% by weightwater and cured for 24 hours at 500° F. The same calcium aluminate wasmixed with ASTM class F fly ash in an amount of 75% by weight of thecalcium aluminate and with water in an amount of 34% by weight ofcalcium aluminate and cured for 24 hours at 500° F. The set samples weretested for compressive strengths in accordance with the above mentionedAPI Specification 10. The set sample formed with calcium aluminate alonehad a compressive strength of only 410 psi while the sample formed withcalcium aluminate and fly ash had a compressive strength of 2120 psi.

[0048] Thus, the present invention is well adapted to carry out theobjects and attain the ends and advantages mentioned as well as thosewhich are inherent therein. While numerous changes may be made by thoseskilled in the art, such changes are encompassed within the spirit ofthis invention as defined by the appended claims.

What is claimed is:
 1. A method of cementing a subterranean zonecomprising the steps of: admixing calcium aluminate, fly ash, sodiumpolyphosphate and sufficient water to form a pumpable slurry; pumpingsaid slurry into the subterranean zone; and allowing said slurry to setinto a hard impermeable mass therein.
 2. The method of claim 1 whereinsaid fly ash is ASTM class F fly ash.
 3. The method of claim 2 whereinsaid ASTM class F fly ash has a Blaine fineness of about 10,585 squarecentimeters per gram.
 4. The method of claim 1 wherein said calciumaluminate and fly ash react to increase the compressive strength of theset slurry.
 5. The method of claim 1 wherein said sodium polyphosphateincludes sodium hexametaphosphate, sodium triphosphate and vitreoussodium phosphates.
 6. The method of claim 1 wherein said water isselected from the group consisting of fresh water and saltwater.
 7. Themethod of claim 1 wherein said slurry further comprises a set retarderfor lengthening the amount of time it takes said slurry to thicken andset.
 8. The method of claim 7 wherein said set retarder is selected fromthe group consisting of citric acid, gluconic acid and tartaric acid. 9.The method of claim 8 wherein said set retarder is present in an amountin the range of from about 0.5% to about 2% by weight of saidcomposition.
 10. The method of claim 1 wherein said subterranean zone isa high temperature subterranean zone containing carbon dioxide.
 11. Themethod of claim 1 wherein said calcium aluminate, fly ash, sodiumpolyphosphate and water are present in an amount sufficient to producesaid hard impermeable mass.
 12. The method of claim 1 wherein said setmass maintains its compressive strength after 28 days in the presence ofa sodium carbonate solution at 600° F.
 13. A method of cementing asubterranean zone comprising the steps of: admixing calcium aluminate,ASTM class F fly ash, sodium polyphosphate and sufficient water selectedfrom the group consisting of fresh water and saltwater to form apumpable slurry; pumping said slurry into the subterranean zone; andallowing said slurry to set into a hard impermeable mass therein. 14.The method of claim 13 wherein said ASTM class F fly ash has a Blainefineness of about 10,585 square centimeters per gram.
 15. The method ofclaim 13 wherein said sodium polyphosphate includes sodiumhexametaphosphate, sodium triphosphate and vitreous sodium phosphates.16. The method of claim 13 wherein said slurry further comprises a setretarder for lengthening the amount of time it takes said composition tothicken and set.
 17. The method of claim 16 wherein said set retarder isselected from the group consisting of citric acid, gluconic acid andtartaric acid.
 18. The method of claim 17 wherein said set retarder ispresent in an amount in the range of from about 0.5% to about 2% byweight of said composition.
 20. The method of claim 13 wherein saidsubterranean zone is a high temperature subterranean zone containingcarbon dioxide.
 21. The method of claim 13 wherein said calciumaluminate, fly ash, sodium polyphosphate and water are present in anamount sufficient to produce said hard impermeable mass.
 22. The methodof claim 13 wherein said set mass maintains its compressive strengthafter 28 days in the presence of a sodium carbonate solution at 600° F.23. A method of cementing a high temperature subterranean zonecontaining carbon dioxide comprising the steps of: admixing calciumaluminate, ASTM class F fly ash, sodium polyphosphate, a retarder andsufficient water selected from the group consisting of fresh water andsaltwater to form a pumpable slurry; pumping said slurry into thesubterranean zone; and allowing said slurry to set into a hardimpermeable mass therein.
 24. The method of claim 23 wherein said ASTMclass F fly ash has a Blaine fineness of about 10,585 square centimetersper gram.
 25. The method of claim 23 wherein said sodium polyphosphateincludes sodium hexametaphosphate, sodium triphosphate and vitreoussodium phosphates.
 26. The method of claim 23 wherein said set retarderis selected from the group consisting of citric acid, gluconic acid andtartaric acid.
 27. The method of claim 26 wherein said set retarder ispresent in an amount in the range of from about 0.5% to about 2% byweight of said composition.
 28. The method of claim 23 wherein saidcalcium aluminate, fly ash, sodium polyphosphate, retarder and water arepresent in an amount sufficient to produce said hard impermeable mass.29. The method of claim 23 wherein said set mass maintains itscompressive strength after 28 days in the presence of a sodium carbonatesolution at 600° F.